Apparatus adapted to apply an electrostatic charge to moving fibrous elements



Sept. 5, 1967 J. E OWENS 3,340,429

APPARATUS ADAPTED TO APPLY AN ELECTRDSTATIC CHARGE TO MOVING FIBROUS ELEMENTS Filed April 4, 1966 2 Sheets-Sheet 1 ZNVENTOR JOHN EDWARD OWENS BY WWW ATTORNEY Sept. 5, 1967 J. E. OWENS 3,340,429

APPARATUS ADAPTED TO APPLY AN ELECTROSTI'ATIC CHARGE TO MOVING FIBROUS ELEMENTS 2 Sheets-Sheet 2 Filed April 4, 1966 T0 SOLVENT RECOVERY FIG-4 POSITIVE 0.0. SOURCE INVENTOR JOHN EDWARD OWENS ATTORNEY United States Patent Hockessin, Del., assignor to E. I. Wilmington, Del.,

This application is a continuation-in-part of United States application Ser. No. 372,652, filed June 4, 1964, and now abandoned, which in turn is a continuation-inpart of United States Patent No. 3,163,753 to Disabato and Owens, dated Dec. 29, 1964.

This invention relates to an apparatus for placing electrostatic charges on dielectric material and particularly to corona discharge means employed in charging continuously forwarded dielectric materials, particularly those materials in the form of continuous filaments and still more especially those of synthetic organic polymeric structure.

It is the object of the present invention to provide improved apparatus for electrostatically charging fibrous webs in a corona discharge field. Another object is to provide a novel and useful ion gun capable of uniform corona discharge. These and other objects will become apparent in the course of the specification and claims.

A corona discharge is the electrical discharge occurring in a gas surrounding a conductor when the potential gradient at a point in a non-uniform electric field exceeds the critical value for ionization causing a local selfsustained discharge. It is a phenomenon particularly associated With sharp-edged electrodes and essentially occurs in a limited region near the electrodes, the rest of the gap carrying a so-called dark current. The magnitude of the electric field gradient required to produce a corona discharge is the same as that required to produce arcing; the difiference between the two being that in corona the critical value has been reached only in a limited region of the breakdown path between electrodes. The apparatus of the present invention is useful in supplying a corona field particularly in applying an electrostatic charge to a fibrous web.

In accordance with the present invention, an apparatus is provided through which a web of fibrous elements is forwarded in a path of advance or flow-path while an electrostatic charge is placed upon the said elements by a corona discharge assembly. The corona discharge assembly consists of an ion gun and a target electrode positioned on opposite sides of and adjacent to the path of advance of the web. Forwarding means are provided to convey the web into the zone of corona influence in order to impose an electrostatic charge upon it. The ion gun of the present invention consists of a row of parallel needles rigidly held in a fixed lateral spacial relationship. In assembly opposite the target electrode, the row of needles is arranged transverse to and completely across the path of advance of the fibrous elements with the points of the needles facing the target electrode. In the ion gun of the present invention each needle is connected to a common source of high voltage current through an individual resistor of relatively high impedance, i.e., at least 1 megohm.

The invention will be more readily understood with reference to the drawings.

FIGURE 1 is a perspective view partially in section of the ion gun of the present invention.

FIGURE 2 is an elevation of a plug-in corona discharge needle of the ion gun of FIGURE 1.

FIGURE 3 is a top view with the top cover removed -velocity vapor currents of a second embodiment of the ion invention.

FIGURE 4 is a cross-sectional elevation indicating schematically the arrangement of the various elements in a system wherein the ion gun of the present invention is useful. This system is described and claimed in US. application No. 372,623 to Hollberg and Owens, filed June 4, 1964.

Referring particularly to FIGURE 1 corona discharge needles 25 are rigidly fixed in supporting face 24 of the ion gun illustrated. Connection of each needle to the main power supply through a port 33 is through a separate lead 32 in series with a resistor 28. The unit illustrated is particularly useful in applications where high are experienced, the curved electrically insulating top 23 and flat electrically insulating face 24 providing a path of low turbulence for such vapor flow. As shown in FIGURE 2, needle 25 has a sharp point 22 and is conveniently of a plug-in variety with a conducting socket attached to prongs 34 adaptable for making electric contact with lead 32. Another embodiment of the ion gun is shown in FIGURE 3, wherein a row of conducting needles is mounted parallel in side 27 of a housing 26 (the top of the housing being removed in the drawing). Each needle is connected through an individual resistor 28 to a conducting bar 29. Terminal 30 is provided along conducting bar 29 for connecting the device to a high voltage source such as 35, shown in FIGURE 4. In operation, the top (not shown) of housing 26 is secured in place and encloses resistor 28 and bar 29. Use of a resistor in series with each needle and the high voltage source provides needle-toneedle uniformity of corona flow which is especially important when operating at a low current per needle such as is necessary in the production of sheet products formed from plexifilaments such as those described in US. Patent No. 3,081,519 to Blades and White. As is degun of the present scribed in greater detail in the aforementioned application to Hollberg and Owens, in the formation of sheet products by electrostatic laydown of the charged plexifilaments of Blades and White, a maximum or peak charge accumulates on the plexifilamentary web as corona current (as indicated by a-target plate current) is increased.

Efiicient operation under these conditions requires that a targetto-ground current no greater than that required to apply the peak charge be employed. Furthermore, operation at corona currents above that required to produce a peak charge on the web results in non-uniformity of sheet product. Generally a corona current at each needle from about 5 to about 30 microamps is adequate; between 10 and 20 microamp per needle is preferred for charging a plexifilamentary web. At this relatively low current per needle small changes in emitted current magnify the eifect of non-uniformity. The ion gun described herein provides a high impedance circuit from the high voltage source to each needle so that normal fluctuations in the effective dynamic resistance of corona discharge have little effect on emitted current. This is done by using a resistance of suflicient magnitude in series with each needle to provide a voltage drop of at least about 3,000 volts. In a typical ion gun/target configuration the eflective dynamic resistance of corona discharge is about 60 megohms, whereas the resistance placed in series with each needle to provide a corona current at each point of at least 5 microamps is typically 600 megohms. Use of the resistors makes the needle-to-needle current variations much less sensitive to such factors as point/target spacing, point wear, point contamination, and point-to-point spacing.

The nature of the target electrode is not critical. It may be a round bar variety as illustrated in US. Patent No. 3,163,753, dated Dec. 29, 1964. Preferably, particularly where high velocity vapor flow accompanies the fibrous web, the fiat target plate described and claimed in combination in the aforementioned application to Hollberg and Owens is preferred. Similarly, while the means for forwarding the fibrous web is not critical, the present invention is particularly useful in electrostatically charging a freshly-spun plexifilamentary web. A suitable systern for this operation is shown in FIGURE 4 which is described in detail below.

Referring particularly to FIGURE 4, a spinneret device 1, is connected to a source ofv polymer dissolved in an organic solvent. Polymer solution 2 under pressure is fed through extrusion orifice 3 into intermediate pressure or let-down pressure zone 4 and then through spinning orifice 5 into web forming chamber 6. The extrudate from spinning orifice 5 is a plexifilament 7. Due to the pressure drop at spinning orifice 5 vaporization of solvent creates a vapor blast which, by virtue of impingement upon baffie 8 concomitantly with plexifilament 7, generally follows the path of advance of the plexifilament 7 from spinning orifice 5 to collecting surface 9, thereby creating a flow pattern within chamber 6 as indicated by the arrows. Bafile '8 is mounted on shaft 10 which in turn is oscillatably mounted in bearing 11 and is powered to oscillate by means not shown. While oscillation of the bafiieis not essential, it is preferred in the spreading of the plexifilament, at the beginning of the path of advance. Target plate 13 and ion gun 14 are disposed on opposite sides of the path of advance of the plexifilament web, downstream from the web forming and spreading devices. Target plate 13 is so disposed that the vapor blast originating at 5 and the air flow pattern in chamber '6 holds plexifilament 7 in brushing contact with its charging surface. Target plate 13 is connected to ground by wire 15 and microammeter 16, the latter indicating target plate current. The position of needles 25 with reference to target plate 13 is important for efficient operation.

It will be apparent that the clearance between the needle points 22 and plate 13 should be as small as efficient operation will permit. Generally a clearance of from about 0.2 to about 2 inches is satisfactory although this will vary with the design and capacity of the particular equipment.

It' should be noted that as the distance between the needle points and the target electrode is reduced, the total applied voltage across the resistor and the gap should be reduced to keep the current per point in the 5 to 30 rnicroampere range. In addition, as the needle point-toelectrode distance is decreased, it is desirable to increase the number of points per inch in order to deposit a uniform charge on all parts of the web of fibrous elements. On the other hand, when increasing the distance between the needle points and the target electrode the voltage should be increased and/ or the number of points per inch decreased. For optimum operation, the points should be equally spaced along the conducting bar.

,It has been found convenient in adjusting positioning of gun 14 opposite to target plate 13 to create a carbon black deposit on target plate 13 by spraying powdered carbon black into the operating area between the plate and the gun. An oval pattern is outlined by carbon deposits opposite each needle indicating the area of electrostatic influence of each needle under the particular conditions employed. The patterns laid down by single points are centered the same distance apart as the needles, are oval shaped, and have a height of about 2.5 cm. and a width of about 0.6 cm. When a flat target plate is employed, smoothest operation of the equipment with uniform laydown occurs when the above-mentioned test patterns are centered at a distance between /2 in. (1.3 cm.) and A in. (1.9 cm.) from trailing edge of the target. Placement of the ion gun at a point further upstream results in pinning or clinging of the web to the target plate. This results in bunching for an instant, an uneven diselectric charge by power charge across the web width, and a falling free of the bunched web to give a non-uniform sheet. In addition, when the gun is aimed further upstream on the target plate, the web charge curve is very abrupt, making the process more difiicult to control. On the other hand, if the ion gun is aimed too near the trailing edge of the target plate, secondary ionization will develop at the edge of the target plate providing charged ions of opposite polarity which will discharge the web unevenly. The web will then collapse and give a ropey strand which in turn gives a non-uniform sheet. In addition the discharged web will not pin well to the belt because of loss of charge.

After passing through the charging zone, plexifilament 7, as shown in FIGURE 4, is deposited upon a collecting surface 9. The surface illustrated is a continuous belt forwarded by drive rolls 36. The belt is given an opposite source 37 to that imposed on plexifilament 7 in the charging zone. Due to differences in their electrostatic charge, the plexifilarnent 7 is attracted to surface 9 and clings to it in its arranged condition as a sheet 38 with suflicient force to overcome the disruptive influences of whatever vapor blast may reach this area. Surface 9 carries sheet 38 out of chamber 6 through port 39. Flexible elements 40 across port 39 and also across port 41, which permits reentry of the unloaded continuous belt, assist in retention of vapor within chamber 6. The sheet is then lightly compacted by compacting roll 44 and is collected on wind up roll 42. A conventional solvent recovery unit 43 may be beneficially employed to improve economic operation.

The details of construction of the ion gun of the present invention may vary widely. A typical gun, of the type shown in FIGURE 1, has 41 needles each attached to a distant 600 megohm resistor 28 by means of a small conducting wire 32. The conducting wires are electrically insulated from one another and were further encased in a bar of polyacetal resin which supports the needles, The

resistors are encased in a box-like housing 26 located outside the vapor stream from the jet. The needles of the ion gun extend 0.48 em. out of the polyacetal resin face 24. The needles are 0.14 cm. in diameter at the shank and .007 cm. in radius at the tip. They are mounted on 0.95 cm. centers apart on the bar. This gun is particularly useful in charging a plexifilamentary web using the apparatus of FIGURE 4. Mounted opposite a flat target plate extending 12.7 cm. along the path of advance and 40.7 cm. across the said path and having a straight, thin trailing edge (to promote smooth vapor flow), it may be conveniently located 3.8 cm. away from the target, with its needles on a line 1.3 cm. above the trailing edge of the target plate.

In an actual embodiment, the gun of the present invention is of a flat face construction at 24 with a streamlined top surface 23 as shown in FIGURE 1. In another embodiment the needles are recessed within shielding or focusing cavities along the gun face. Furthermore, while the gun has been described as particularly useful in the charging of plexifilamentary webs, it is broadly applicable to the charging of all filamentary materials, particularly man-made filaments of a continuous structure.

Many equivalent modifications of the above invention will be apparent to those skilled in the art without a departure from the inventive concept.

What is claimed is:

1. An apparatus adapted to apply an electrostatic charge to fibrous elements, said apparatus comprising, in combination, means for forwarding the said fibrous elements in a linear path, a corona discharge device consisting of an ion gun and a grounded target electrode therefore disposed on opposite sides of the said path, the said ion gun being comprised of a row of parallel conducting needles fiXed rigidly in a fixed lateral spacial relationship arranged transverse to and completely across the said path with the points of the said needles facing the said target electrode, a direct current source of high voltage power, and means connecting each said needle to said source of high voltage, each connecting means including a high impedance resistor, the said voltage and resistor providing a corona current at each needle of from about 5 to about 30 microamps, each said resistor providing a voltage drop of at least about 3,000 volts.

2. The apparatus of claim 1 wherein the said needles protrude from a flat faced supporting housing having a streamlined surface disposed toward said forwarding means.

3. The apparatus of claim 1 wherein the said forwarding means is a vapor blast.

4. The apparatus of claim 2 wherein said housing includes a dielectric bar as its fiat face, said bar being provided with a plurality of longitudinally aligned sockets for receiving said needles.

5. The apparatus of claim 1 wherein is provided a support for said resistors, spaced laterally from said linear path.

6. The apparatus of claim 4 wherein is provided a support for said resistors located at one end of said bar.

References Cited UNITED STATES PATENTS 15 J. A. SILVERMAN, Assistant Examiner.

MILTON O. HIRSHFIELD, Primary Examiner. 

1. AN APPARATUS ADAPTED TO APPLY AN ELECTROSTATIC CHARGE TO FIBROUS ELEMENTS, SAID APPARATUS COMPRISING, IN COMBINATION, MEANS FOR FORWARDING THE SAID FIBROUS ELEMENTS IN A LINEAR PATH, A CORONA DISCHARGE DEVICE CONSISTING OF AN ION GUN AND A GROUNDED TARGET ELECTRODE THEREFORE DISPOSED ON OPPOSITE SIDES OF THE SAID PATH, THE SAID ION GUN BEING COMPRISED OF A ROW OF PARALLEL CONDUCTING NEEDLES FIXED RIGIDLY IN A FIXED LATERAL SPACIAL RELATIONSHIP ARRANGED TRANSVERSE TO AND COMPLETELY ACROSS THE SAID PATH WITH THE POINTS OF THE SAID NEEDLES FACING THE SAID TARGET ELECTRODE, A DIRECT CURRENT SOURCE OF HIGH VOLT- 